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EMSL ON THE ROAD TO AGU!

Learn more about EMSL research by attending EMSL-related presentations and poster sessions at AGU and by visiting the EMSL booth, number 345.

Presentations at AGU

• Abstract
  GCS is one of the most promising ways of mitigating atmospheric greenhouse gases. Mineral carbonation reactions are potentially important to the long-term sealing effectiveness of caprock but remain poorly predictable, particularly reactions occurring in low-water supercritical CO₂(scCO₂)-dominated environments where the chemistry has not been adequately explored. In situ probes that provide molecular-level information is desirable for investigating mechanisms and rates of GCS mineral carbonation reactions. MAS-NMR is a powerful tool for obtaining detailed molecular structure and dynamics information of a system regardless whether the system is in a solid, a liquid, a gaseous, or a supercritical state, or a mixture thereof. However, MAS NMR under scCO₂ conditions has never been realized due to the tremendous technical difficulties of achieving and maintaining high pressure within a fast spinning MAS rotor. In this work, we report development of a unique high pressure MAS NMR capability, and its application to mineral carbonation chemistry in scCO₂ under geologically relevant temperatures and pressures. Our high pressure MAS rotor has successfully maintained scCO₂ conditions with minimal leakage over a period of 72 hours. Mineral carbonation reactions of a model magnesium silicate (forsterite) reacted with 96 bars scCO₂ containing varying amounts of H₂O (both below and above saturation of the scCO₂) were investigated at 50°C. Figure 1 shows typical in situ ¹³C MAS NMR spectra demonstrating that the peaks corresponding to the reactants, intermediates, and the magnesium carbonation products are all observed in a single spectrum. For example, the scCO₂ peak is located at 126.1 ppm. Reaction intermediates include the aqueous species HCO³- (160 ppm), partially hydrated/hydroxylated magnesium carbonates (166-168 ppm), and can easily be distinguished from final product magnesite (170 ppm). The new capability and this model mineral carbonation process will be overviewed in light of fundamental geochemical science needs for GCS implementation.

• Abstract
  A new X-ray computed tomography capability for flow and transport research of geologic cores at the pore scale is now available to users at the U.S. Department of Energy's EMSL, a national scientific laboratory located at Pacific Northwest National Laboratory. The new capability consists of a NIKON Metris 225-320 LC with three interchangeable static and rotating targets generating variable 225-320 kV X-ray energies and spot sizes between 3 and 10 microns. This system was specifically designed to image the pore structure and connectivity of large diameter cores of loosely consolidated sediments typical of the vadose zone. The high energies of the system will permit CT imaging of cores up to 15 cm in diameter with a spatial resolution between 12 and 75 microns dependent on the diameter of the core. Examples of time-lapse imaging will be presented as well dual energy capability for differentiating air versus fluid filled pores. Additional in situ tomography capabilities will be demonstrated, and the EMSL user access via peer-review proposal process will be discussed.

Poster Sessions at AGU

• Abstract
  Field measurements of secondary organic aerosol (SOA) find significantly higher mass loads than predicted by models, sparking intense effort that is focused on finding additional SOA sources, but leaves many of the fundamental assumptions that are used by models unchallenged. Current air-quality models use absorptive partitioning theory assuming SOA particles are liquid droplets that form instantaneous reversible equilibrium with gas phase. Further, they ignore the effects of adsorption of spectator organic species during SOA formation on SOA properties and fate. Using an accurate and highly sensitive experimental approach for studying evaporation kinetics of size-selected single SOA particles, we characterized room-temperature evaporation kinetics of laboratory generated α-pinene SOA and ambient atmospheric SOA. The experimental setup was first tested by measuring the evaporation kinetics of single component organic particles of known vapor pressure. We show that, as expected for liquid droplets, smaller particles evaporate faster, and that these data yield the correct vapor pressure. We then study the evaporation kinetics of α-pinene SOA and find that evaporation proceeds in two stages: a fast stage, during which 50% of the particle volume evaporates in ~100 minutes, followed by a slower stage, when additional 25% evaporate in 1400 minutes, which is in sharp contrast to the ~10 minutes timescale predicted by current kinetic models. α-pinene SOA formed in the presence of "spectator" hydrophobic organic vapors like dioctyl phthalate, dioctyl sebacate, pyrene, or their mixture, were shown to adsorb noticeable amounts of these organics, forming what we term here 'coated' SOA particles. We show that these adsorbed coatings reduce evaporation rates of SOA particles. Moreover, aging of coated SOA particles dramatically reduces evaporation rates, and in some cases nearly stops it. For example, aging of SOA with adsorbed pyrene reduces evaporation rate to the point that only ~11% of the particle volume evaporates within 24 hrs. For all cases studied in this work, SOA evaporation behavior is size-independent and does not follow the evaporation kinetics of liquid droplets, which is in sharp contrast with model assumptions. To address the question of how closely the laboratory observations described above reflect reality in the atmosphere we characterized the evaporation kinetics of size-selected atmospheric SOA particles sampled in-situ during the recent Carbonaceous Aerosols and Radiative Effects Study (CARES) field campaign. We find that the evaporation of ambient SOA is very similar to that of coated and aged laboratory-generated α-pinene SOA. Ambient SOA particles in Sacramento, CA lose between 17% and 25% of their volume in 6 hours. Like laboratory SOA, their evaporation is size-independent and does not follow the kinetics of liquid droplets. The findings about SOA phase, evaporation rates, and the importance of spectator gases and aging – all indicate the need to reformulate the way SOA formation and evaporation are treated by models.

• Abstract
  Capture and sequestration of anthropogenic CO₂ in depleted oil and gas reservoirs, unminable coal seams and deep saline aquifer are being intensively studied as a promising strategy to mitigate CO₂ emission into the atmosphere. Two critical research areas are trapping mechanisms and caprock sealing efficiency, which are controlled by interfacial processes at the fluid-fluid and fluid-rock interfaces. Fundamental understanding of capillary/viscous effects and host rock heterogeneity on trapping mechanisms and sealing efficiency can be gained through micromodel pore-scale displacement experiments. Experimental capabilities are being developed at the Environmental Molecular Sciences Lab (EMSL), Pacific Northwest National Laboratory (PNNL) to study coupled flow and reactive transport processes in complex systems at the pore-scale at relevant storage pressures and temperatures. This presentation highlights our strategies for design and construction of a unique high-pressure system for micromodel experimentation and a capability for visualizing dynamic interfacial processes using a solvatochromic dye under supercritical conditions. Preliminary results on displacement of brine by supercritical CO₂ will be presented.

• Abstract
  In the recent years pore-scale models emerged as a powerful tool to study reactive transport in porous media. The pore-scale models can employ varies numerical methods for solving conservation laws governing flow and transport on the pore-scale but they have a common feature: their discrete approximations are systems of ordinary differential equations (ODEs) which can contain an enormous number of unknowns ($ >10^{10}$) when applied to a computational domain on the scale of REV. A direct solution of these ODEs can be extremely expensive. This necessitates development of advanced algorithms for model (or dimension) reduction. We developed a novel dimension reduction method for large size ODE systems. The method can significant accelerate pore-scale simulations regardless of the nature of a numerical solver. The method relies on a computational closure of averaged evolution balance equations. The computational closure is achieved via short bursts of a pore-scale model conducted in small portions of the computational domain. The dimension reduction model was used to simulate flow and transport with mixing controlled reactions and mineral precipitation. The good agreement with micro-fluidic experiments and analytical solutions confirms the accuracy and computational efficiency of the dimension reduction model.

• Abstract
  Capture and storage of carbon dioxide in deep geologic formations represents one promising scenario for minimizing the impacts of greenhouse gases on global warming. The ability to demonstrate that CO₂ will remain stored in the geological formation over the long-term is needed in support of widespread implementation decisions, and knowledge of mineral-fluid chemical transformation rates is an essential aspect. The majority of previous research on mineral-fluid interactions has focused primarily on the reactivity of minerals in aqueous solutions containing various amounts of dissolved CO₂. Long-term caprock integrity, however, could also be dictated by mineral transformations occurring in low-water environments dominated by the supercritical CO₂ (scCO₂) fluid phase, which is expected to slowly displace or dessicate residual aqueous solution at the caprock-fluid interface. Many of the mechanisms of mineral interfacial reactions with hydrated or water-saturated scCO₂ are unknown and there are unique challenges to obtain kinetic and thermodynamic data for mineral transformation reactions in these fluids. We are developing a high-pressure atomic force microscope (AFM) that will enable in-situ, atomic scale measurements of metal carbonate nucleation and growth rates on mineral surfaces in contact with hydrated scCO₂ fluids. This apparatus is based on the hydrothermal AFM that was developed by Higgins et al.¹, but includes some enhancements and is designed to handle pressures up to 100 bar. The noise in our optically-based cantilever deflection detection scheme is subject to perturbations in the density (due to index of refraction dependence) of the compressible supercritical fluid. Consequently, variations in temperature and pressure within the fluid cell are a primary technical challenge with possible significant impact in imaging resolution. We demonstrate with our test fluid cell that the equivalent rms noise in the deflection signal is similar to (and in some cases less than) the equivalent noise for an AFM in its 'standard configuration' under controlled pressures of ~80 bar and temperatures of 60-80°C and therefore in-situ atomic scale imaging of mineral surfaces in scCO₂ should be possible. Single, monatomic steps on the surface of (101_4) calcite under nitrogen at 83 bar and at room temperature have been imaged with this new high pressure apparatus. This talk will also focus on recent progress in the development of this instrumentation, which will enable a unique platform for elucidating the role of water in mineral carbonation reactions in scCO₂, providing a means for determining effective kinetic constants.

   

  1. Higgins SR, CM Eggleston, KG Knauss, CO Boro. 1998. "A hydrothermal atomic force microscope for imaging in aqueous solution up to 150°C." Review of Scientific Instruments 69(8):2994-2998.

• Abstract
  Immiscible fluids displacement in porous media affects several processes in the subsurface: geological carbon sequestration, enhanced oil recovery, and groundwater contamination by nonaqueous phase liquids. Characterization of immiscible displacement processes at the microscopic level (i.e., pore-scale) is important to better understand macroscopic processes (i.e., continuum-scale). A series of displacement experiments was conducted to investigate the impacts of viscous and capillary forces on fluid distribution in a silicon-based homogeneous pore network micromodel. Seven wetting-nonwetting fluid pairs with viscosity ratios (M = log(μw/μnw)) ranging 4 orders of magnitude were studied: the micromodel was initially saturated with a wetting fluid, and displaced by a nonwetting fluid under different flowrates (characterized by the capillary number, Ca = μnw*unw/σow, ranging 3 orders of magnitude). Nonwetting fluid saturation was measured using fluorescent microscopy. Results showed two distinctive fingering mechanisms associated with viscosity ratio at low capillary number: viscous fingering occurs when M is below -0.9, and capillary fingering when M is above -0.5. Stable displacement occurred at higher capillary numbers, and this depends on M. Under the same Ca number, nonwetting fluid saturation increased from viscous fingering to capillary fingering, which supports numerical modeling study of Lenormand at al. (1985, J. Fluid Mech.). Results from this study also showed there is a linear correlation between interfacial area (at) and nonwetting fluid saturation, and at between immiscible fluids at low viscosity ratio is 5% higher than at high viscosity ratio.

• Abstract
  Soil desiccation is a potentially robust remediation process to slow migration of inorganic or radionuclide contaminants through the unsaturated zone. The application of gas-phase partitioning tracer tests has been proposed as a means to estimate initial water volumes and to monitor the progress of the desiccation process at field sites. Tracer tests have been conducted in porous medium columns with various water saturations using sulfur hexafluoride as the conservative tracer and tricholorofluoromethane and difluoromethane as the water-partitioning tracers. For porous media with minimal silt and/or organic matter fractions, tracer tests provided reasonable saturation estimates for water contents in dry materials. However, for sediments with considerable silt and/or organic matter fractions, tracer tests only provided satisfactory results when water contents were at least 0.03 - 0.05, depending on the porous medium. For dryer conditions, the apparent tracer retardation increases due to direct air–soil sorption, which is not included in traditional retardation coefficients derived from advection-dispersion equations accounting only for air–water partitioning and water–soil sorption. Based on these results, it is suggested that gas-phase partitioning tracer tests may be used to determine initial water contents in sediments, provided the initial water saturations are sufficiently large. However, tracer tests are not suitable for quantifying moisture content during and after the desiccation process when water contents are expected to be low.

• Abstract
  A new microfluidics capability for flow and transport research at the pore scale is now available to users at the U.S. Department of Energy's EMSL, a national scientific laboratory located at Pacific Northwest National Laboratory. The new capability consists of a state-of-the-art microfabrication clean-room and a dedicated laboratory for micromodel flow and transport experimentation. Micromodels are two-dimensional representations of porous media etched in into silicon wafers, glass, polymers, or natural sediments. Pore sizes are typically on the order of tens of microns, but can be configured to be both smaller and larger. Fluid injection occurs with low-pulsation, high-precision pumps. Images are obtained using fluorescence and Raman microscopy. The new capability permits simultaneous spatial and time-resolved spectroscopic examination of physical and chemical processes. It also has the potential to address fundamental scaling issues associated with fluid flow and reactive transport from both a combined experimental and theoretical approach. In this presentation, the new microfabrication and micromodel experimental capabilities will be demonstrated, and the EMSL user access process will be discussed. Several examples of recent experiments will be presented.

Visit the EMSL booth

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Come meet EMSL experts at booth number 345 right across from the Springer booth. We will be on hand to talk to you about the wide variety of world-class capabilities in geochemistry and biogeochemistry at EMSL, including advanced tools for imaging, spectroscopy, microfluidics, and genomics studies. Ask us how to access these capabilities and collaborate with our internationally recognized experts, at no cost, via our peer-reviewed proposal process.